It is known to produce dense membranes by means of solvent casting which involves forming a solution of a polymer comprising a surface active agent and casting it onto a liquid support to produce a thin layer which is subsequently dried (by evaporation of the solvent present in the polymer solution) to form a solid, dense membrane. The applied solvent is, however, generally substantially immiscible with the liquid support in order to avoid a reduction of the surface tension of the liquid support wich could lead to instability of the developing membrane and possible generation of holes therein.
It would be advantageous to be able to use a solvent which is substantially soluble in the liquid support without simultaneous generation of undesired holes despite substantially reduced interfacial tension, thus shortening the membrane solidification time substantially because not all solvent would have to be removed from the membrane forming layer by means of evaporation.
Surprisingly, it has now been found that dense (that is non-porous) membranes can be produced, starting from a solution comprising a polar polymer and/or a polar prepolymer in a polar solvent which is substantially soluble in the polar liquid used as support.